I know the dealer took the tail off to place a new impeller... since then I have this problem...

put it in gear and turn the prop by hand, then wedge a lump of wood between the prop and housing and try to turn the flywheel, you shouldn't be able to turn the flywheel i'm more inboard than out board but i know on some engines that the input shaft has a friction welded joint, i'm wondering if this is the case with yours and it's parted, you've going to have to take the gearbox off i fear to find out what this other guy has done